42 EPITHELIUM AND ENDOTIIELIUM. 



distance from each other. Amongst those of the first form, 

 some possess a shallow constriction ; in some the constricting 

 furrow is so deep, that the two portions of the cell are con- 

 nected by a short bridge, which in others is reduced to a slender 

 filament. The division of the nucleus is not always into two; 

 it i/ not uncommon to find cells whose nucleus is rosette-shaped. 

 Further, we meet with numerous large, flat cells, belonging to 

 the most superficial layers, in whose interior is a yacuole of 

 variable size, and shut up in this a young brood of from two 

 or three, to eight or ten cells. This variety of proliferation is 

 known as endogenous. 



Epithelium of the Bladder. As we have already re- 

 marked, the epithelium of the mucous membrane of the 

 urinary bladder of mammals is laminated and transitional. A 

 thin shred from the internal surface of the urinary bladder 

 of the rabbit, guineapig, dog, or cat, in the fresh state, may 

 be covered in half per cent, solution of common salt in water, 

 or in a bichromate solution. If the bladder has been kept from 

 twent3^-four to forty -eight hours in the latter liquid, specimens 

 are obtained in which the following appearances may be ob- 

 served: Firstly, large pavement cells, bounded b} r a double 

 contour, and consisting of a uniformly granular protoplasm 

 which contains from two to five clear vesicular nuclei, each 

 with a double contour, and possessing a large, shining nucleo- 

 lus. In these pavement cells-we see that, as a rule, only one 

 of the surfaces is even ; that, namely, which corresponds to 

 the free surface of the mucous membrane. Of this we may 

 convince ourselves by examination of connected masses of 

 epithelium or of vertical sections. The deep surface of each 

 cell is marked by depressions with prominent ridges between 

 them, and is that by which it is in contact with the club- 

 shaped or conical cells of the subjacent layer, so that the 

 rounded summits of the latter fit into the depressions of the 

 former. The cells of the second layer consist of a uniformly 

 granular protoplasm, have a double contour membrane, and 

 each contains an oval vesicular nucleus, and within this a 

 shining iiucleolus. They possess simple or divided processes 

 of varying length and thickness. Among lliem there are 

 spindle-shaped cells which insinuate themselves between the 

 processes of the former layer. 



To study the single layer of epithelium of the urinary 

 bladder of the frog, consisting as it does of large granular 

 cells, we spread upon a slide a portion of this organ with the 

 free surface upwards, and cover it with a piece of thin glass, 

 on the under surface of which a small drop of half per cent, 

 solution of common salt has been placed. Wherever folds 

 occur in the mucous membrane the epithelial cells show them- 



